I think plant identification questions are a perfect fit for an SE site! They easily conform to the question/answer format, and I completely agree that there's a market for them.
The botany proposal you linked to has been deleted, and unfortunately I don't have enough understanding of the system to know why. You could always propose a site dedicated just to plant identification, no matter where or what the plant is, or its purpose. I'd support it, but I don't know if it would come to fruition.
In the meantime, if you have those questions, Gardening & Landscaping is definitely a place to consider posting them. I'm very active there and can confirm that many plant identification questions, even if they're not about a garden plant, are acceptable, and generally well-received. A significant number of users really enjoy answering identification questions in general, and they don't have to relate to gardening.
If you check out the flower or plant identification questions, you'll find that people frequently ask for more information. They may want more details and pictures, which might not always be possible if the plant was just found in the woods. In that case, identification can be more of a stab in the dark, but that doesn't mean they don't want the question.
I recently was involved in a meta discussion which was a bit different but led to the same conclusion. The moderator who answered it pointed me to some random identification questions that had nothing to do with gardening specifically. They are here, here, and here. Another example is this, which is just seeking an identification of a tree that was seen in a park someplace.
The Great Outdoors also welcomes some identification questions, and that might be a better fit for the types of things people see outdoors that are unrelated to gardening. I've asked some questions like that about trees and animals. Others have asked about mushrooms. They're also seeking a larger diversification of questions. Plant identification questions were not specifically on that list, but it would be worth asking on their meta or in their chat room.